1. Field of the Invention
The present invention relates to low dropout voltage regulators, and in particular, to those built in biCMOS and CMOS processes.
2. Description of the Related Art
Low dropout voltage regulators (LDOs) are used in power supply systems to provide a regulated voltage at a predetermined multiple of a reference voltage. LDOs have emerged as front-line integrated circuits (ICs) in the last decade, being used in palmtop and laptop computers, portable phones, and other entertainment and business products. Due to the growing need to save power, all battery-operated electronic systems use or will probably use LDOs with low ground current. More and more LDOs are built in bipolar complementary metal oxide semiconductor (biCMOS) and enhanced CMOS processes, which may provide a better, but not always cheaper product.
FIG. 1 is a simplified block diagram of a conventional CMOS low dropout positive voltage regulator LDO 10, which is based on FIGS. 2 and 3 of U.S. Pat. No. 5,563,501 (Chan). An unregulated input voltage VIN is applied to an input terminal 12. A bandgap reference 14 delivers a desired reference voltage to an inverting input line 16 of an error amplifier 18, which is an operational transconductance amplifier (OTA). A non-inverting input line 20 of the amplifier 18 is connected to the output of a negative feedback network (resistors R130 and R232). An output line 21 of the error amplifier 18 is coupled to the input of a buffer 22.
The buffer 22 in FIG. 1 is a voltage follower with an output stage (M24, M25, Q17, Q18 in FIG. 4 of U.S. Pat. No. 5,563,501) that provides a low output impedance to line 23, which is coupled to a high parasitic capacitance gate of a power p-channel metal oxide semiconductor (PMOS) path transistor 24 (path element). The power transistor 24 has its drain connected to an output terminal 26, where a regulated output voltage VOUT is available. The feedback network (R130 and R232) is a voltage divider, which establishes the value of VOUT. The feedback network consists of an upper resistor R130 connected between the output rail 26 and a node N1, and a lower resistor R232 connected between node N1 and a ground terminal 28.
As described in U.S. Pat. No. 5,563,501 (col. 1), a desirable LDO may have as small a dropout voltage as possible, where the xe2x80x9cdropout voltagexe2x80x9d is the voltage drop across the path element (power PMOS transistor 24 in FIG. 1), to maximize DC performance and to provide an efficient power system. To achieve a low dropout voltage, it is desirable to maximize the channel-width-to-channel-length ratio of the power PMOS transistor 24, which leads to a larger area and a large parasitic capacitance between gate and drain/source of the power PMOS transistor 24. Such large PMOS transistors, having a large parasitic capacitance between the gate and the drain/source, makes frequency compensation more difficult, affecting the transient response and permitting a high frequency input ripple to flow to the output.
Being a negative feedback system, an LDO needs frequency compensation to keep the LDO from oscillating. The LDO 10 in FIG. 1 performs frequency compensation by using an internal Miller compensation capacitor 34, which is connected through additional circuitry 36 between the output terminal 26 and line 21. In U.S. Pat. No. 5,563,501, the additional circuitry 36 is a current follower. The frequency compensation arrangement of the LDO 10 in FIG. 1 permits the use of a single, low-value external capacitor 40, having a low equivalent series resistance (ESR) 42, which may be intrinsically or externally added.
The buffer 22 in FIG. 1 is built using a foldback cascode operational amplifier with NPN input transistors and an NPN common-collector output stage. However, these NPN transistors are not available in standard digital N-well CMOS processes.
In another LDO disclosed in U.S. Pat. No. 6,046,577 (Rincon-Mora), the buffer 22 is built in a biCMOS process using two cascaded stages: a common-collector NPN voltage follower and a common-drain PMOS voltage follower.
G. A. Rincon-Mora discloses another solution for the buffer 22 in a paper entitled xe2x80x9cActive Capacitor Multiplier in Miller-Compensated Circuits,xe2x80x9d IEEE J. Solid-State Circuits, vol. 35, pp. 26-32, January 2000, by replacing the first NPN stage with a common-drain NMOS, thus being closer to a CMOS process. Nevertheless, in order to eliminate the influence of bulk effects on the NMOS stage (for N-well processes), which affects power supply rejection ratio (PSRR), additional deep n+ trench diffusion and buried n+ layers are needed.
The frequency compensation used in the Rincon-Mora paper mentioned above is the same as that disclosed in U.S. Pat. No. 6,084,475 (Rincon-Mora), and is close to that of FIG. 1. The difference is that the Miller compensation capacitor 34 is connected between the output terminal 26 and an internal node of the error amplifier 18, as shown by the dotted line in FIG. 1. In this configuration, no additional circuitry 36 is needed.
The LDOs described above have several drawbacks, including: (1) the use of expensive biCMOS or enhanced CMOS processes, (2) limited closed-loop bandwidth, e.g., under 100 KHz, which may be caused by the output stage (M24, M25, Q17, Q18 in FIG. 4 of U.S. Pat. No. 5,563,501) in the buffer 22 of FIG. 1 or caused by other circuit elements, (3) non-ideal transient response, even at low ESR, due to a low slew-rate (SR) (maximum possible rate of change) provided for the internal capacitor 34 and/or due to the output stage (M24, M25, Q17, Q18 in FIG. 4 of U.S. Pat. No. 5,563,501) in the buffer 22 of FIG. 1 or due to other circuit elements, and (4) poor power supply rejection ratio (PSRR)(rejection of noise) at high frequency. Some of these limitations are disclosed in Rincon-Mora""s paper (see FIGS. 7 through 9).
A low dropout voltage regulator with non-Miller frequency compensation is provided in accordance with the present invention. The low dropout voltage regulator comprises a first operational transconductance amplifier (OTA), a second OTA, a power p-channel metal oxide semiconductor (PMOS) transistor, and a feedback network. The first OTA has an inverting input, a non-inverting input and an output. The inverting input is coupled to a voltage reference circuit. The non-inverting input is coupled to a feedback network. The first OTA is configured to operate as an error amplifier. The second OTA has an inverting input, a non-inverting input and an output. The non-inverting input is coupled to the output of the first OTA. The output of the second OTA is coupled to the inverting input of the second OTA to form a voltage follower.
The power PMOS transistor has a source terminal, a drain terminal and a gate terminal. The source terminal is coupled to an input voltage terminal. The gate terminal is coupled to the output of the second OTA. The drain terminal is coupled to an output voltage terminal. The feedback network comprises a first resistor, a second resistor, and a frequency compensation capacitor. The first and second resistors are coupled in series between the output voltage terminal and a ground terminal. The frequency compensation capacitor is connected in parallel with the first (upper) resistor of the feedback network. The non-inverting input of the first OTA is coupled to a first node between the first and second resistors.
In order to optimize frequency compensation and transient response, by eliminating the need for a Miller compensation capacitor, both OTAs are designed with wide-band and low-power (low-current) circuit techniques. These wide-band, low-power OTAs enable the use, in addition to the single frequency compensation capacitor, of a single, low-value load capacitor with a low intrinsic equivalent series resistance (ESR).
Some conventional LDOs need high-value, externally-added ESRs to become stable. An LDO using a high-value ESR has the main disadvantage of a poor transient response: strong undershooting and overshooting. The LDO circuit according to the present invention may use the frequency compensation of a voltage regulator where the ESR specification does not exist, i.e., a voltage regulator with a simple load capacitor without an additional, external ESR and without choosing a particular type of load capacitor with a high intrinsic ESR over a temperature domain. In one embodiment, an LDO is stable with small and inexpensive load capacitors having a typical value of a few xcexcF.
All parasitic poles from the signal path may be pushed to higher frequencies, producing a desired quasi single-pole behavior (the frequency response of a circuit may be characterized by poles and zeroes in a transfer function in the complex frequency s-domain).
To enhance the PSRR of the LDO according to the invention, the first wide-band OTA (error amplifier) may have a cascode second stage biased from the reference voltage, and the second OTA may have an additional PMOS transistor.
In one embodiment, a high efficiency LDO according to the invention may be advantageously built in a standard digital CMOS process, which allows lower manufacturing costs. A xe2x80x9cstandard digital CMOS processxe2x80x9d is a CMOS technology process that provides standard NMOS and PMOS transistors without any specific enhanced properties. Any additional components (such as resistors, capacitors, etc.) in the circuit can be implemented using the same processing steps as implementing the standard NMOS and PMOS transistors. The standard digital CMOS process may be referred as an N-well CMOS technology, which does not require additional processing steps. In contrast, the biCMOS process (referred to in U.S. Pat. Nos. 5,563,501 and 6,046,577) and the enhanced CMOS process require additional processing steps, such as additional deep n+ trench diffusion and buried n+ layer (referred to in the above-referenced article xe2x80x9cActive Capacitor Multiplier in Miller-Compensated Circuitsxe2x80x9d). The biCMOS process and the enhanced CMOS process are more expensive to use than a standard digital CMOS process. In other embodiments, the LDO according to the invention may be built in biCMOS or enhanced CMOS processes.
In one embodiment, an LDO according to the invention has an enhanced transient response closer to an ideal response, without using known Miller-type frequency compensation techniques. The enhanced transient response is due to a higher closed-loop bandwidth at maximum current, and elimination of an internal Miller capacitor.
In one embodiment, an LDO according to the invention has good PSRR at high frequency, due to the wide-band techniques and the lack of Miller-type frequency compensation.
Another aspect of the invention relates to a method of regulating an input voltage. The method comprises receiving an input voltage at a source terminal of a power p-channel metal oxide semiconductor (PMOS) path transistor; producing an output voltage at a drain terminal of the power PMOS transistor; comparing a reference voltage with a part of the output voltage; amplifying a difference between the part of the output voltage and the reference voltage; controlling a gate terminal of the power PMOS transistor in response to the amplified difference between the part of the output voltage and the reference voltage; and performing a non-Miller compensation, so that when a low-value, low intrinsic equivalent series resistance (ESR) load capacitor is coupled to the drain terminal, a behavior close to a single-pole loop, delivering a step and an almost undershoot and overshoot-free load transient response, is achieved.